The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Referring now to FIG. 1, an amplifier 10 includes a first module 12 that can be characterized by a first transfer function z=f1(x), where x is an independent variable such as frequency, current, voltage, and the like, and z is an output signal. The amplifier 10 includes a second module 14 that can be characterized by a second transfer function y=f2(z), where y is a dependent variable such as frequency, current, voltage, and the like. At least one of the first function f1 and the second function f2 may be non-linear and/or dependent on a third independent variable, such as time, temperature, and the like. Therefore, it can be challenging to calibrate or otherwise predict an overall total transfer function y=f(x) of amplifier 10 as the third variable changes.
Referring now to FIG. 2A, a programmable gain amplifier 20 is shown that behaves in accordance with generic amplifier 10 of FIG. 1. A programmable current source 22 corresponds to first module 12 and generates a programming current Igm based on an input signal Vcontrol. A differential amplifier 24 corresponds to second module 14 and has a gain A=vout/vin that is based on the programming current Igm. FIG. 2B shows a functional block diagram of programmable gain amplifier 20.
Referring now to FIG. 2C, a first plot 30 shows an unscaled characteristic curve of programmable current source 22 and a second plot 32 shows an unscaled characteristic curve of differential amplifier 24. First plot 30 shows that if all other variables are held constant, then the programming current Igm is more sensitive to temperature changes at higher temperatures T than at lower temperatures T. Second plot 32 shows that if all other variables are held constant, then the gain A is more sensitive to temperature changes at lower temperatures than at higher temperatures. The opposite characteristics of these temperature sensitivities can make it challenging to control the gain of amplifier 20 as its temperature varies.